[mikeash]

There's no ambiguity only because nobody has bothered to mount conventional weapons on ICBMs. There's nothing preventing that, and indeed the US has seriously considered it as a way to implement Prompt Global Strike. Those plans have been held back by the fear that launching such a weapon could be mistaken for a nuclear launch, of course.

Is the problem with hypersonic weapons just that it looks like the players won't make that same choice?

[breatheoften]

Based on the arguments in this thread -- I don't see the difference with respect to m.a.d. either.

HS launch could carry conventional or nuclear threat, it's not possible to know which after detection of launch. For mad, we must always treat these launches as nuclear -- and therefore in the end, no one uses these weapons (unless they are intending to launch global nuclear war)...?